Switch with surge protection

ABSTRACT

An insulating member of metal oxide varistor material separates the two conductive parts of a hermetically sealed enclosure to which the electrodes of the switch are attached. The metallic oxide varistor material has a voltage versus current characteristic such that when normal voltage appears across the electrodes, a high impedance is presented by the insulating member and when voltages in excess of normal voltage appears across the electrodes, a rapidly decreasing impedance is presented by the insulating member, thereby limiting the voltage which is sustainable across the electrodes of the switch.

United States Patent Harnden, Jr.

[ 1 Jan. 16, 1973 SWITCH WITH SURGE PROTECTION lnventor: John D. l-larnden, Jr., Schenectady,

Assignee: General Electrlc Company Filed: Dec. 22, 1971 Appl. No.: 210,979

US. Cl. ..200/166 C, 200/l68 G, 200/222 Field of Search ..H0lh/l/60; 200/222, 166 C, 200/168 G References Cited UNITED STATES PATENTS l2ll942 5/1962 l2/l959 l/l966 Gerisch ..200/222 Cook ..200/222 Cook et al. ..200/222 Cook et al. ..200/222 X 6/l967 Cook et al. ..200/222 l2/l968 Coutant .200/222 Primary Examiner-H. 0. Jones Attorney-John F. Ahern ct al.

[ ABSTRACT An insulating member of metal oxide varistor material separates the two conductive parts of a hermetically sealed enclosure to which the electrodes of the switch are attached. The metallic oxide varistor material has a voltage versus current characteristic such that when normal voltage appears across the electrodes, a high impedance is presented by the insulating member and when voltages in excess of normal voltage appears across the electrodes, a rapidly decreasing impedance is presented by the insulating member, thereby limiting the voltage which is sustainable across the electrodes of the switch.

9 Claims, 14 Drawing Figures VOLTS PER MILLIMETER PATENTEUJAH 16 I975 SHEET 1 BF 2 TIT HHHI llIHHI IlllHlI IIHIIII HHHHI I l l IIIHI 10- 10' no" AMPERES PER sou WEF if; l8? gigb I I0 CENTIMETER PATENTED JAN 16 I973 SHEET 2 BF 2 SWITCH WITH SURGE PROTECTION The present invention relates in general to electrical switches and in particular to button-type switches in which is included electrical protection for devices such as lamps and appliances connected in circuit with such switches from surge and other spurious electrical voltages.

Electrical switches, commonly referred as button switches, because of their button-like appearance are used as wall switches to provide electrical power to the lamps, appliance motors and the like. One such switch is a mercury button switch in which pools of mercury are utilized to make and break electrical contact. The pools are located in a pair of chambers separated by an apertured partition within the switch housing. In one position, or orientation about a rotational axis, the pools of mercury are separated by the partition. In another axial position, an aperture in the partition is positioned to allow the pools of mercury to merge, thereby providing conductive connection between a pair of terminals conductively juxtaposed to the pools of mercury. Such switches have a number of advantages such as quiet operation, minimal actuating force, and are compact and small in size. ln some applications, such switches may present a disadvantage. The quick breaking action of the switch produces high induced voltages in the circuits in which the switch is connected, particularly in circuits with significant inductance. For example, in ordinary house wiring, when the only inductive load connected in circuit is the inductance of the house wiring, the rapid action of the mercury button switch may produce voltages in the circuit as high as twice the normal operating voltages. Of course, when appliances and devices which have higher inductance are connected in circuit, the rapid breaking of the circuit thereof by the switch produces much higher inductive voltages. Voltages in excess of 2000 volts have been measured under such circumstances. Such voltages have an adverse effect on the electrical devices connected to the house lines such as filamentary lamps, clock motors and the like. Increasing the operating voltage on filamentary lamps rapidly deteriorates the performance thereto and reduces the life thereof as well. Also, high inductive voltages may cause the insulation of such devices as clock motors to fail.

Accordingly, an object of the present invention is to provide a switch of the kind described in which the magnitude of the electrical surges produced thereby is substantially reduced.

Another object of the present invention is to provide fast acting switches in which the amplitude of the voltage surge produced thereby is limited to a predetermined value.

Another object of the present invention is to provide a switch for electrical devices which in addition to providing fast action, also provide protection against electrical pulses without the need for separate or additional elements to provide such function.

A further object of the present invention is to provide a switch of long life.

In carrying out the present invention, in one illustrative embodiment thereof, there is provided a sealed enclosure having a pair of conductive portions and an insulated member including an annular portion. The annular portion of the insulating member has a peripheral outer portion contacting one of the conductive portions and has an inner peripheral portion contacting the other of said conductive portions. A pair of electrodes are provided each included within the enclosure and each connected to a respective one of the conductive portions. The conductive portions include externally located portions providing terminals for connecting the switch in circuit. The annular portion of the insulating member is constituted of a metal oxide varistor material having an alpha in excess of 10 in the current density range of from 10 to 10 ampcres per square centimeter. The spacing of the peripheral portions are set so that a high impedance is presented between the peripheral portions when normal voltages appear therebetween and when voltages in excess of normal voltages progressively appear thereacross, a rapidly decreasing impedance is presented by the annular portion in accordance with the alpha of a material thereof, thereby limiting the variation in the voltage between the peripheral portions.

The features of the invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings in which,

FIG. I is a perspective view of a mercury button switch made in accordance with the present invention,

FIG. 2 is a sectional view of the switch shown in FIG.

FIG. 3 is an end view of the switch of FIG. 1,

FIG. 4 shows graphs of the electrical characteristics of three materials of differing voltage gradients and alphas suitable for utilization in the switches of the present invention,

HO. 5 is a side view in section of another embodiment of the present invention,

FIG. 6 is an end view ofthe embodiment of FIG. 5,

FIG. 7 is a side view in section of another embodiment of the present invention,

FIG. 8 is a side view in section of a further embodiment of the present invention,

H08. 9, l0 and 11 are broken perspective views of the ceramic barrier of the switch shown in FIG. 1 useful in explaining the operation of the mercury button switch.

FIG. 12 is a still further embodiment of the present invention showing a sectional view of a vacuum button switch.

FIG. 13 is one end view of the device of FIG. 12.

FIG. 14 is another end view of the device of FIG. 12.

The present invention is directed to providing improvements in switches such as the mercury button switches described in US. Pat. No. 3,229,354 Cook et a]. Referring to FIGS. l, 2 and 3 of the drawing, there is shown a mercury button switch comprising a cylindrical shell 11 or first conductive portion with a closed end 12 and an open end having an outwardly extending flange 13 with a lip 14. The other solid elements of the switch comprise a terminal metal pin 15 or second conductive portion centrally positioned with respect to a retaining ring 16 by means of an insulating glass seal 17 of annular form. Directly bonded to the glass seal is a cylindrical ceramic barrier or liner 18 which has an aperture (not shown) through which mercury can flow as will be explained below. When direct-bonded, the pin 15, glass seal 17, retaining ring 16, and barrier 18 form an assembly which is sealed, as by welding of the ring 16 to the flange 13 of the shell with the barrier on the interior thereof. Bonded to the exterior portion of the annular glass seal is an annular member 20 of metal oxide varistor material.

Reference is now made to FIGS. 9, l and 11 which show the barrier or liner l8 and the manner of its operation in the switch 10. The ceramic barrier or line 18 has a transverse central partition 25 which serves to divide the interior of the switch into pools of mercury 26 and 27. The partition 25 has a circular opening or port 28 and a reservoir cavity 29 in one side thereof. The through opening 28 is designed to permit the mercury on both sides of the partition to flow together at the center of the opening and complete an electrical circuit between the metal shell 11 and the head 23 of the terminal pin 15. it is important that the switch be capable of operation with a rotational angle of 20 or less. Accordingly, the through opening is located as far as possible from the geometric axis of the metal shell. The reservoir or cavity 29 is positioned with its lower edge on a line drawn through the center axis of the shell and the center axis of the through opening 28. Referring particularly to FIGS. and 11, as the switch is rotated in a counter clockwise direction, as indicated by arrows 24, the through opening will begin to sink below the top level of the mercury pools 26 and 27. This begins to remove the obstruction between the two pools and they enter the through opening 28 with rounded frontal surfaces 30 and 31. As these two bodies of liquid mercury come into contact, a high inrush current will immediately vaporize the frontal surfaces of the mercury bodies. Final circuit enclosure might occur only after two or more such vaporizations or explosions. The purpose of the reservoir 29 is to dump the last traces of mercury into the mercury stream at the opportune moment when the two pools begin to merge at the center of the through opening 28. This action increases the kinetic energy of the moving mercury bodies and minimizes the local explosive effects of the contacting surfaces of mercury. When the switch is turned to open circuit position, the reservoir 29 again serves to increase the kinetic energy of the moving mercury bodies as they begin to separate. The metal shell 11 has an indexing notch 19 which is util ized for positioning and rotating the switch. A complementary notch on the surface of the ceramic barrier 18, insures the proper orientation of the barrier 18 within the metal shell 11. Prior to the final sealing of the pin 15, ring 16 and barrier 18 assembly and mercury within the shell H, a non-oxidizing atmosphere is provided within the switch. Preferably, this atmosphere is predominantly argon with hydrogen added for alternating current operation. For direct current operation, the atmosphere is preferably predominantly hydrogen and both alternating and direct current operation may be at a pressure above atmospheric, for example, 40 pounds per square inch.

In operation, a switch handle (not shown) which, however, may be part of an assembly such as shown in the above-mentioned U.S. Pat. No. 3,229,354, is seated on the outer cylindrical side of the shell 11 in the indexing notch 19 and actuated to impart rotary motion within a restricted arc to the mercury button switch. Rotation of the switch produces the opening and closing of the electric circuit from the terminal pin 15 to the shell 11 by means of the mercury pools 26 and 27 as described above with reference to FIGS. 9, l0 and l l.

The annular member 20 of metal oxide varistor material has a peripheral outer portion 21 and an inner peripheral portion 22. The insulating glass seal member 17 has an exterior oriented surface which is recessed from the plane of the exterior side of the retaining ring 16 to form a recess into which the annular member 20 of metal oxide varistor material fits. Annular member 20 is bonded to the glass seal 17. The outer peripheral portion 21 of the metal oxide varistor member is metallized and conductively secured to the inner surface of the annular retaining ring 16. The inner peripheral portion 22 of the metal oxide varistor member is also metallized and bonded to the terminal portion of the pin 15.

The annular member 20 is constituted of a metal oxide varistor material such as described in Canadian Pat. No. 831,691, which has a nonlinear voltage versus current characteristic. The metal oxide varistor material described in the aforementioned patent is constituted of fine particles of zinc oxide with certain additives which have been pressed and sintered at high temperatures to provide a composite body or wafer of material. The current versus voltage characteristics of the composite body is expressed by the following equation:

I (V/C), l where V is voltage applied across a pair of opposed surfaces or planes,

1 is the current which flows between the surfaces,

C is a constant which is a function of the physical dimensions of the body as well as its composition and the process used in making it,

a is a constant for a given range of current and is a measure of the nonlinearity of the current versus voltage characteristic of the body.

In equation (1), when V is used to denote voltage between opposed planes of a unit volume of material, or voltage gradient, current flow through the unit volume of material in response to the voltage gradient becomes current density. For the metal oxide varistor material for current densities which are very low, for example, in the vicinity of a microampere per square centimeter, the alpha (a) is relatively low, i.e., less than 10. in the current density range of from 10' to ID amperes per square centimeter, the alpha is high, i.e., substantially greater than 10 and relatively constant. In the current density range progressively in excess of IO amperes per square centimeter, the alpha progressively decreases. When the current versus voltage characteristic is plotted on log-log coordinates, the alpha is represented by the reciprocal of the slope of the graph in which current density is represented by the abscissa and voltage gradient is represented by the ordinate of the graph. For a central range of current densities of from l0 to It) amperes per square centimeter, the reciprocal of the slope is relatively constant. For current densities below this range, the reciprocal of the slope of the graph progressively decreases. Also for the current densities above this range, the reciprocal of the slope of the graph progressively decreases.

The voltage gradient versus current density characteristics of three types of material in log-log coordinates are set forth in FIG. 3. Graphs 35 and 36 are materials of high voltage gradient material and graph 37 is a graph of low voltage gradient material. For all of the graphs in the current density range from to 10 amperes per square centimeter, the alpha is high and is substantially greater than l0 and relatively constant. For current densities progressively greater than 10 amperes per square centimeter, the alpha progressively decreases. For current densities progressively less than H) amperes per square centimeter, the alpha also progressively decreases.

As the metal oxide varistor material is a ceramic material, the surfaces thereof may be metallized for facilitating electrical connections thereto in a manner similar to the manner in which other ceramic materials are metallized. For example, Silver Glass Frit, duPont No. 7713, made by the duPont Chemical Company of Wilmington, Delaware, may be used. Such material is applied as a slurry in a silk screening operation and fired at about 550C to provide a conductive coating on the surface. Other methods such as electroplating or metal spraying could be used as well.

The nonlinear characteristics of the material results from bulk phenomenon and is bi-directional. The response of the material to steep voltage wave fronts is very rapid, Accordingly, the voltage limiting'effect of the material is practically instantaneous. Heat generation occurs throughout the body of material and does not occur in specific regions thereof as in semiconductor junction devices, for example. Accordingly, the material has good heat absorption capability as the conversion of electrical to thermal energy occurs throughout the material. The specific heat of the material is 0.12 calories per degree Centigrade per gram. Accordingly, on this account, as well, heat absorption capability of the material is advantageous as a surge absorption material.

The material, in addition to the desired electrical and thermal characteristics described above, has highly desirable mechanical properties. The material has a fine grain structure, may be readily machined to a smooth surface and formed into any desired shape having excellent compressive strength. The material is readily molded in the process of making it. Accordingly, any size or shape of material may be readily form ed for the purposes desired.

The closed portion 12 ofthe shell I] of the switch of FIGS. I, 2 and 3, serves as one electrode or terminal of the switch and the exterior terminal portion of the pin 15 serves as the other electrode of the switch. When the switch is opened, the voltage of the line source would appear across the outer peripheral portion 21, and the inner peripheral portion 22 of member 20. Accordingly, to provide an appropriate low current drain through the member under normal operating voltages for the switch, the metal oxide varistor material with the appropriate voltage gradient versus density characteristics is selected. The normal operating voltage rating of the switch of FIGS. 1, 2 and 3 may be altered without use of a different material by the provision of electrodes which extend along one of the opposed surfaces of the wafer toward one another, thereby applying a higher voltage gradient to the wafer for a given applied voltage between the electrodes such as shown in FIGS. 5 and 6.

Reference is now made to FIGS. 5 and 6 which show a modification of the switch of FIGS. 1, 2 and 3. The elements of the embodiment of FIGS. 5 and 6 which are identical to the elements of an embodiment of FIGS. 1, 2 and 3 are identically designated. The embodiment of FIGS. 5 and 6 include an outer conductive ring 40 and inner conductive ring 41 useful in setting the voltage versus current characteristics of the metal oxide varistor material connected in shunt between the electrodes 12 and 15 of the switch. The outer ring 40 is substantially a flat ring having a circular inner edge 42. The outer conductive ring is bonded to the retaining ring 16 and to the external surface of the member 20 of the metal oxide varistor material. Similarly, the inner conductive ring 41 has an outer edge 43 which is circular in outline and is bonded along one face thereof to the external face of the member 20 of metal oxide varistor material and the inner edge of the ring 40 is bonded to the terminal pin 15. The inner edge 42 of the outer conductive ring 40 and the outer edge 43 of the inner conductive ring 41 are spaced so that the uniform distance exists therebetween around the periphery thereof. The spacing of the inner edge of the outer ring and the outer edge of the inner ring is set to produce the desired voltage versus current characteristics in the material. The closer the spacing, the lower the voltage at which substantial increase current flow is obtained.

Reference is now made to FIG. 7 which shows a sectional view of another embodiment of the present invention. The elements of the embodiment of FIG. 7 which are identical to the elements of the embodiment of FIGS. 1 and 2 are identically designated. In the embodiment of FIG. 7, the glass seal member 17 has been replaced with an annular insulating member 50, made of metal oxide varistor material to which the pin I5 is bonded. The annular retaining ring 16 is bonded to an outer peripheral portion of the member 50. In the switch structure of FIG. 7, the element 50, constituted of metal oxide varistor material, provides both the sealing and voltage limiting functions for the switch and simplifies the construction and the processing of the switch.

Reference is now made to FIG. 8 which shows another embodiment of the present invention. The elements of FIG. 8 identical to the elements of FIGS. 1, 2 and 3 are similarly designated. In this figure, members 20 of metal oxide varistor material secured to the glass seal 17 has been eliminated and the barrier or liner member 18 thereto is constituted of metal oxide varistor material and is designated barrier 55. As the geometrical form of the barrier 55 is identical to the geometrical form of barrier I8, the same numerals are utilized to designate the geometric elements thereof. The spacing of the bottom of the well 29 is set in respect to the opposed face of the central partition 25 so that a high impedance is presented between such faces when normal voltages appear on the switch terminals and hence between the pools of mercury 26 and 27. When voltages in excess of normal voltage appear thereacross as occurs on opening of the switch, a

rapidly decreasing impedance is presented across the partition in accordance of the alpha of the material of the partition 25. Accordingly, the variation in voltage between the pools of mercury 26 and 27 and hence between the terminals of the switch is limited.

Reference is now made to FlGS. I2, 13 and 14 which show another embodiment of the present invention. The switch 60 includes a cup shaped enclosure member 6] ofa conductive material having an opening 62 in the base portion 63 thereof and an open portion opposite the base portion on which circular flange 64 is located. An insulating member 65 of metal oxide varistor material annular in form is provided in which the outer peripheral portion 66 thereof is secured to the one end of the member 61 and in which the inner peripheral portion 67 thereof is conductively secured to a conductive pin 68 having a cylindrical terminal portion 69 and a contact element 70. A flexible bellows member 71 of metallic material such as phosphor bronze, for example, circular in outline, and having a conductive contact element 72 secured conductively at a central region thereof is provided. The outer peripheral portion of the bellows member 7! is conductively secured to the flange 64 of the cup member 61 by welding, so as to register axially the contact element 72 with respect to the contact element 70 of the pin. The sealed enclosure may be evacuated or may be backfilled with an appropriate gas such as hydrogen or nitrogen, for example, if desired. For this purpose, a tubulation 75 is provided in the side cylindrical surface of the cup member 61. The contact elements may be made of any of a number of materials such as copper alloys, nickel alloys, molybdenum, tungsten and the like.

The aperture 62 in the base of the cup is circular in outline and the terminal portion 63 to which the inner peripheral surface of the metal oxide varistor 67 material is secured, is centrally located so that the distance between a radial point on the pin 68 and a radial point on the inner edge 62 of the aperture is identical in order to ensure that all of the metal oxide varistor material is utilized in effecting a voltage limiting and electrical energy absorption function as explained above.

The switch of FIGS. l2, l3 and 14 described above may be actuated manually or by means of any of a number of mechanical assemblies well-known to those skilled in the art. In the embodiment of FIGS. l2, l3 and 14, the bellows element 71 is flexible and in the absence of force supplied to the contact element 72 is spaced from contact element 70 when a mechanical force is applied between the contact elements 70 and 72 to overcome the spring force of the bellows, the contact elements engage. Upon removal of the force, the contact elements return to their disengaged positions.

While the invention has been described in the specific embodiments, it will be appreciated that modifications may be made by those skilled in the art and I intend by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What 1 claim as new and desire to secure as Letters Pat. of the U.S. is:

1. A switch comprising:

a sealed enclosure having a pair of conductive portions and an insulating member including an annular portion,

said annular portion of said insulating member having a peripheral outer portion contacting one of said conductive portions and an inner peripheral portion contacting the other of said conductive portions,

a pair of spaced electrodes each included within said enclosure and each connected to a respective one of said conductive portions,

externally located portions of said conductive portions providing terminals for connecting said switch in circuit,

the annular portion of said insulating member being constituted ofa metal oxide varistor material.

2. The combination of claim 1 in which said metal oxide varistor material has an alpha in excess of 10 in the current density range of 10 to 10 amperes per square centimeter.

3. The combination of claim 1 in which the portions of said annular portion of said insulating member in contact with said conductive portions are spaced to provide a current flow between said conductor portions which is low when normal operating voltage appears across said conductive portions and when voltages in excess of normal voltage progressively appear thereacross a rapidly decreasing impedance is presented by said annular portion of said insulating member in accordance with the alpha of the material, thereby limiting the voltage appearing between said conductive portions.

4. The combination of claim 1 in which said outer and inner peripheral portions of said annular portion are cylindrical.

5. The combination of claim 1 in which said annular portion of said insulating member is provided with a side surface which is substantially planar, a conductive ring concentrically located on said side surface, said conductive ring being in conductive contact with one of said conductive portions, the edge of said conductive ring adjacent the other of said conductive portions being uniformly spaced with respect thereto in said side surface.

6. The combination of claim 1 in which said annular portion of said insulating member is provided with a side surface which is substantially planar, an outer conductive ring and an inner conductive ring concentrically located on said side surface, said outer conductive ring in conductive contact with said one conductive portion, said inner conductive ring in conductive contact with said other conductive portion, the adjacent edges of said ring being uniformly spaced in said side surface.

7. The combination of claim 1 in which said'annular portion of said insulating member has a pair of major opposed faces, a layer of metal oxide varistor material bonded to one of said faces and outer peripheral por tion of said layer conductively connected to said one conductive portion and an inner peripheral portion of said layer conductively connected to the other of said conductive portions, said layer being constituted of a metal oxide varistor material having an alpha in excess of l0 in the current density range of from l0 to l0 amperes per square centimeter, the spacing of said peripheral portions being set so that high impedance is presented between said peripheral portions when normal voltages appear between said peripheral portions and when voltages in excess of normal voltage progressively appear thereacross, a rapidly decreasing impedance is presented by said layer in accordance of the alpha of the material thereof, thereby limiting the variation in voltage between said conductive portions.

8. The combination of claim 1 in which said sealed enclosure includes a resilient portion for enabling said electrodes to be brought into contact with one another.

9, A switch comprising:

a cylindrical metal shell having an open end and a closed end,

a cover plate hermetically sealed to close said open end, said plate including an annular insulator having a terminal pin sealed to an inner peripheral portion thereof and a metal retaining ring sealed to the outer peripheral surface thereof, said retaining ring conductively sealed to said shell,

an insulating liner in said shell having a transverse partition dividing said shell into two chambers,

said chambers containing a quantity of mercury 0ccupying less than half the volume thereof,

said partition having an opening therethrough for the passage of mercury between said chambers, said partition having a well for mercury diametrically opposite said opening, said partition being constituted of a metal oxide varistor material having an alpha in excess of 10 in the current density range of from 10 to 10 amperes per square centimeter, the spacing of the bottom of said well in respect to the opposed surface of said partition being set so that a high impedance is presented therebetween when normal voltages appear between the pools of mercury in said chambers and when voltages in excess of normal voltage progressively appear thereacross, a rapidly decreasing impedance is presented by said partition in accordance with the alpha of the material of said partition, thereby limiting the variation in voltage between the the pools of mercury. 

1. A switch comprising: a sealed enclosure having a pair of conductive portions and an insulating member including an annular portion, said annular portion of said insulating member having a peripheral outer portion contacting one of said conductive portions and an inner peripheral portion contacting the other of said conductive portions, a pair of spaced electrodes each included within said enclosure and each connected to a respective one of said conductive portions, externally located portions of said conductive portions providing terminals for connecting said switch in circuit, the annular portion of said insulating member being constituted of a metal oxide varistor material.
 2. The combination of claim 1 in which said metal oxide varistor material has an alpha in excess of 10 in the current density range of 10 3 to 102 amperes per square centimeter.
 3. The combination of claim 1 in which the portions of said annular portion of said insulating member in contact with said conductive portions are spaced to provide a current flow between said conductor portions which is low when normal operating voltage appears across said conductive portions and when voltages in excess of normal voltage progressively appear thereacross a rapidly decreasing impedance is presented by said annular portion of said insulating member in accordance with the alpha of the material, thereby limiting the voltage appearing between said conductive portions.
 4. The combination of claim 1 in which said outer and inner peripheral portions of said annular portion are cylindrical.
 5. The combination of claim 1 in which said annular portion of said insulating member is provided with a side surface which is substantially planar, a conductive ring concentrically located on said side surface, said conductive ring being in conductive contact with one of said conductive portions, the edge of said conductive ring adjacent the other of said conductive portions being uniformly spaced with respect thereto in said side surface.
 6. The combination of claim 1 in which said annular portion of said insulating member is provided with a side surface which is substantially planar, an outer conductive ring and an inner conductive ring concentrically located on said side surface, said outer conductive ring in conductive contact with said one conductive portion, said inner conductive ring in conductive contact with said other conductive portion, the adjacent edges of said ring being uniformly spaced in said side surface.
 7. The combination of claim 1 in which said annular portion of said insulating member has a pair of major opposed faces, a layer of metal oxide varistor material bonded to one of said faces and outer peripheral portion of said layer conductively connected to said one conductive portion and an inner peripheral portion of said layer conductively connected to the other of said conductive portions, said layer being constituted of a metal oxide varistor material having an alpha in excess of 10 in the current density range of from 10 3 to 102 amperes per squAre centimeter, the spacing of said peripheral portions being set so that high impedance is presented between said peripheral portions when normal voltages appear between said peripheral portions and when voltages in excess of normal voltage progressively appear thereacross, a rapidly decreasing impedance is presented by said layer in accordance of the alpha of the material thereof, thereby limiting the variation in voltage between said conductive portions.
 8. The combination of claim 1 in which said sealed enclosure includes a resilient portion for enabling said electrodes to be brought into contact with one another.
 9. A switch comprising: a cylindrical metal shell having an open end and a closed end, a cover plate hermetically sealed to close said open end, said plate including an annular insulator having a terminal pin sealed to an inner peripheral portion thereof and a metal retaining ring sealed to the outer peripheral surface thereof, said retaining ring conductively sealed to said shell, an insulating liner in said shell having a transverse partition dividing said shell into two chambers, said chambers containing a quantity of mercury occupying less than half the volume thereof, said partition having an opening therethrough for the passage of mercury between said chambers, said partition having a well for mercury diametrically opposite said opening, said partition being constituted of a metal oxide varistor material having an alpha in excess of 10 in the current density range of from 10 3 to 102 amperes per square centimeter, the spacing of the bottom of said well in respect to the opposed surface of said partition being set so that a high impedance is presented therebetween when normal voltages appear between the pools of mercury in said chambers and when voltages in excess of normal voltage progressively appear thereacross, a rapidly decreasing impedance is presented by said partition in accordance with the alpha of the material of said partition, thereby limiting the variation in voltage between the the pools of mercury. 